Ink jet printing is a non-impact method that produces droplets of ink that are deposited on a substrate such as paper or transparent film in response to an electronic digital signal. Thermal or bubble jet drop-on-demand ink jet printers have found broad application as output for personal computers in the office and the home.
In existing thermal ink jet printing, the printhead typically comprises one or more ink jet ejectors, such as disclosed in U.S. Pat. No. 4,463,359, and each ejector includes a channel communicating with an ink supply chamber, or manifold, at one end and an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels at a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink rapidly bulges from the nozzle and is momentarily contained by the surface tension of the ink as a meniscus. This is a very temporary phenomenon, and the ink is quickly propelled toward a print substrate. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation from the nozzle of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity for propelling the ink droplet in a substantially straight line direction towards a print substrate, such as a piece of paper. Because the droplet of ink is emitted only when the resistor is actuated, this type of thermal ink-jet printing is known as "drop-on-demand" printing. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224, 4,532,530, and 5,281,261, the disclosures of which are totally incorporated herein by reference. Other types of ink-jet printing, such as continuous-stream or acoustic, are also known.
In an ink jet printing apparatus, the printhead typically comprises a linear array of ejectors, and the printhead is moved relative to the surface of the print sheet, either by moving the print sheet relative to a stationary printhead, or vice-versa, or both. In some types of apparatuses, a relatively small ink jet printhead moves across a print sheet numerous times in swathes to form a line image. A partial (e.g. checkerboard) or a desired line image can be produced on the print sheet in each swath. After each line image is completed, the print sheet is advanced and the process is repeated until the entire image is printed. This type of ink jet printing is known as multiple pass (multi-pass) printing or checkerboard printing.
In some cases, an array of ejectors is formed by butting together several printheads, forming a printhead bar. This increases the number of ink jet nozzles so that the printing speed can be increased. The length of the printhead bar may cover only a part of the width of the print substrate. This type of ink jet printhead is called a partial-width printhead. The partial-width printhead can be used in the aforementioned multiple pass printing or checkerboard printing with increased print speed.
Alternatively, a printhead which consists of an array of ejectors and extends to the full width of the print substrate may be used. Ink can be deposited onto the print substrate one line at a time by the full-width array printhead until full-page images are completed. This is called a single pass method of printing. The ink jet printer which uses one or more full-width array printheads is known as a "full-width array" (FWA) printer. When the full-width array printhead and the print substrate are moved relative to each other, image-wise digital data is used to selectively activate the thermal energy generators in the printhead over time so that the desired image will be created on the print substrate. Several full-width array ink jet printheads can be employed in a multi-color ink jet printing system.
With the demand for higher resolution printers, the nozzles of printheads, partial-width printheads and full-width array printheads in ink jet printers are decreasing in size. Nozzle openings are typically about 50 to 80 micrometers in diameter or size for 300 spot per inch (spi) printers. With the advent of high resolution (e.g., &gt;360 spi, including 400 spi and 600 spi) printers, these nozzle openings in a printhead are typically about 10 to about 49 micrometers in diameter or size. These high resolution printheads, partial-width printheads, and full-width array printheads with small nozzle dimensions or sizes require special inks that do not easily clog the small openings. These special inks are more difficult to make than the ink jet inks used in low resolution printheads (e.g. .ltoreq.360 spi), which have less stringent requirements.
One of the critical requirements for an ink jet ink is the ability of the ink to remain in a fluid and jettable condition in a printhead opening which is exposed to air. Latency is the maximum idling time that still allows a printhead to function without failure at 15% relative humidity (RH), jetting an ink with a speed .gtoreq.5 m/s (equivalent to an ink traveling a distance of 0.5 mm in .ltoreq.100 microseconds) after a period of non-use or idling. Long latency is required in order to reduce maintenance of the printhead, especially when there are some infrequently utilized nozzles. A major concern with all ink jet printheads is plug formation or clogging of nozzles, both during operation and between operations of the ink jet printhead. Plug formation is caused by evaporation of water or an organic solvent from the opening of the nozzle. In dye-based inks, this can cause crystallization or precipitation of soluble components such as dye or solid additives as well as an increase in the viscosity of the ink composition. In pigment based inks, this evaporation can cause precipitation of the pigment particles, flocculation or aggregation of the pigment particles, or precipitation of solid ink additives, as well as an increase in the viscosity of the ink composition.
Initial evaporation generally causes an increase in viscosity which affects the ability of the printhead to fire a drop of ink through a nozzle. Some additives have been developed which reduce the rate of evaporation from the ink. However, these additives do not totally eliminate the problem of evaporation from the ink, and, thus, clogging of the nozzles remains a problem, especially with regard to pigment based inks and printheads with small nozzle openings.
The inception of plug formation may cause distortion of the image or alphanumeric characters. This may appear as a drop of ink which is displaced from its intended position. Sometimes two ink drops will be formed equally spaced from the intended target position. Sometimes small numerous satellite drops are produced. On some occasions, the drop may even reach its intended position but at a lower drop volume, producing a lower optical density image. Ultimately, the plugged nozzle will fail to fire and no image will be generated.
Ink jet printers are normally designed to prevent excessive evaporation of water and humectant or solvent from printhead nozzles by sealing the printhead in an air-tight chamber when not in use. These devices may become ineffective with continued printer use because dried ink deposits can be formed at the front face of a printhead and on the rubber seals, causing the system to lose its air-tight condition. However, the system may still be used in an ink jet printer to slow down ink evaporation at the printhead nozzles. Another device used to prevent clogging of the nozzle is a flexible wiper that removes solids formed near or at the opening surface of a printhead nozzle. This device provides necessary printhead cleaning, but it alone may also be ineffective because of the depth of the plug or because of the hardness of the plug, which may be sufficient to resist mechanical removal. Another method of removing ink nozzle clogging is the use of forced air or vacuum suction to clear any deposits from the nozzle. Although these devices may be inefficient and add considerable expense to the cost of the printer, they are sometimes useful for cleaning ink jet printheads.
Another commonly used method to avoid the problem of clogging is to clear the nozzle by firing the printhead in a non-image mode, e.g., into an ink collection receptacle. While this solution is an effective remedy, it requires that the ink form a soft or non-cohesive plug for easy removal. To make this non-image clearance process effective, the surface of the ink in the nozzle must be mechanically or cohesively weak for easy jetting or ink removal. Such a process is set forth in U.S. Pat. No. 5,205,861 to Matrick.
Though periodic firing of ink in an ink jet printhead for maintenance purposes is well known, frequent firing can cause significant loss of valuable ink, thereby raising costs. Therefore, there is a need to develop an ink jet ink which requires less frequent maintenance procedures such as wiping, suction, and ink firing.
Another important requirement for ink jet inks, especially for pigment based inks, is for the pigment particles to remain stable and uniformly dispersed in the ink throughout the life of the ink jet cartridge. Dye-based ink jet inks suffer from deficiencies in water fastness, smear resistance and light fastness after being printed on various substrates. Pigment-based ink jet inks provide an image on a wide variety of substrates, having high optical density and sharp edges with very good water fastness, smear resistance and light fastness. Therefore, pigments are a preferred alternative to dyes in an ink jet ink, provided the pigment particle dispersions can be made stable to prevent undesired flocculation and/or aggregation and settling. Some co-solvents that are good as clogging inhibitors may cause destabilization of pigment particle dispersions and, therefore, cannot be used in pigmented inks. Therefore, proper selection of a desired solvent or humectant for a pigmented ink is extremely important. This is particularly true for a pigmented ink which is used in a high resolution printhead. Thus, there is a need to obtain desired humectants and solvents for pigmented inks with improved stability and jetting performance.
Great effort has been made in attempts to provide ink jet inks having high pigment or pigment and dye loading with acceptable latency and stability, which are required for proper jetting of the ink jet ink composition. However, inks having all of the above-mentioned desirable characteristics have not been easily obtained. Many commercial pigment inks are not stable and form aggregations or precipitations, causing clogging of the printhead. Unstable pigment inks usually have poor latency and jetting performance. However, many dye based inks are claimed to have good properties and performance. For example, U.S. Pat. No. 4,840,674 to Schwarz describes an ink jet ink having sulfolane in combination with dyes and other ink additives without pigments. U.S. Pat. No. 5,169,437 to You discloses water based dye ink compositions exhibiting reduced crusting, clogging and kogation. The subject matter of these patents is incorporated herein by reference.
Moreover, certain ink jet printers require ink jet inks having higher loading of pigments to provide sufficient optical density in a single pass printing method, i.e., without applying additional ink to the substrate or paper using two or more passes. Additionally, certain ink jet printheads in printers are designed to provide enhanced resolution such as, for example, a printhead capable of jetting inks at 400 or 600 spi, as compared to the currently used 300 spi printheads in many commercial ink jet printers. These novel high resolution ink jet printing devices require specially refined inks that do not cause clogging or plugging of the ink jet nozzles, which are significantly narrower than those of .ltoreq.360 spi printheads. In particular, nozzle openings are typically about 50 to 80 microns in diameter or size for .ltoreq.360 spi printheads, and about 10 to 49 microns in diameter or size for &gt;360 spi printheads. Because of the narrower nozzle opening diameters or sizes of high resolution ink jet printers, it is easier for the nozzles to clog. Therefore, there is a need to develop newly advanced inks to address the problem.
Some of the commercial ink jet inks, such as the HP 1200C black ink and Lexmark 1361400 waterproof black ink do not use a dispersant such as a polymer of an aldehyde derivative and a naphthalene sulfonate salt. Further, they contain reasonably low amounts of pigment, having a pigment loading of about 3-5% by weight or less, and produce an optical density .ltoreq.1.0 on some plain papers in a single ink application. Therefore, they require multiple passes to form images with low resolution (.ltoreq.360 spi) printers. These inks are stable, but do not provide a sufficiently high optical density or the needed latency in a single pass method for high resolution (&gt;360 spi) and high speed printing on various substrates, possibly due to low ink coverage, small ink spot size, and inadequate ink/printhead compatibility.
Efforts to increase pigment loading of ink jet inks using commercial pigments and pigment dispersions sometimes have resulted in improperly processed inks having undesirable instability (increased agglomeration or flocculation). Moreover, the decap time or latency of such inks in a high resolution printhead is often very low (usually .ltoreq.10 seconds). Likewise, many commercial ink jet inks (e.g. HP 1200C carbon black and cyan dye inks), which are useful for .ltoreq.360 spi ink jet printing, have a short latency (.ltoreq.10 sec.) when they are used in conjunction with a high resolution (&gt;360 spi) printhead with a nozzle opening diameter or size ranging from about 10 to 49 microns. Accordingly, such inks are not suitable for high resolution ink jet printers because they do not have proper jetting characteristics. Further, many ink jet ink compositions used for low resolution or slow speed printing have an undesired particle size distribution or large pigment particles (&gt;3.0 microns) which are unstable and can cause clogging of the smaller printhead nozzles used in high resolution or high speed printing.
Thus, there is a need in the art for developing new aqueous ink jet ink compositions that can be utilized in high resolution printheads for ink jet printers. Additionally, there is a need for pigmented inks that provide long latency and also remain stable throughout the life of the ink jet printing cartridge. There is a further need for pigmented inks which have good waterfastness, proper particle size and stable pigment particle dispersions which do not cause undesirable nozzle clogging. There is also a need for pigmented inks that can provide high optical density for printing in both single pass (for high speed printing) and multiple pass methods.
Furthermore, there is a need to provide pigmented inks which are capable of printing at high speed or high resolution with a high jetting frequency response, for example, greater than 2.0 KHz, and preferably greater than 3.0 KHz.